Controlled evaporation in heat exchange zones

ABSTRACT

A method for improving the Delta T distribution of a heat exchange zone used in cryogenic separation. The invention comprises refluxing selected hydrocarbon ranges from a rich gas effluent to control the evaporation of the rich gas stream passing through said heat exchange zone.

United States Patent Inventor Ernest A. Harper Bartlesville, Okla. 819,223

Apr. 25, 1969 Sept. 21, 1971 Phillips Petroleum Company Appl. No. Filed Patented Assignee CONTROLLED EVAPORATION 1N HEAT EXCHANGE ZONES Claims, 5 Drawing Figs.

U.S. Cl 208/340, 62/28, 62/26, 62/40 Int. Cl F] 3/00, F25j 3/02, F25j 3/06 Field of Search 62/ 1 1, 22,

HEAT EXCHANGER NITROGEN [56] References Cited UNITED STATES PATENTS 1,768,827 7/1930 Carney 208/340 2,294,547 9/1942 Gerhold 208/340 3,028,332 4/1962 Forbes 208/340 3,181,307 5/1965 Kuerston 62/40 3,293,869 12/1966 Karbosky 62/23 3,512,368 5/1970 Harper 62/27 Primary Examiner-Norman Yudkoff Assistant ExaminerArthur F. Purcell Attorney-Young and Quigg ABSTRACT: A method for improving the AT distribution of a heat exchange zone used in cryogenic separation. The invention comprises refluxing selected hydrocarbon ranges from a rich gas effluent to control the evaporation of the rich gas stream passing through said heat exchange zone.

HELIUM 129 LEAN GAS FEED l I RESIDUE GAS RICH GAS SEPARATOR l6 SEPARATOR I5 HELIUM SEPARATOR l8 RECYCLE HEAT EXCHANGER SEPARATOR lg TEMPERATURE F PATENTEU SEP21 I97l SHEET 3 OF 4 BTU HEAT EXCHANGE INVENTOR.

E. A. HARPER BY W Z/ A T TORNE VS TEMPERATURE F SHEET 0F 4 BTU HEAT EXCHANGE INVENTOR.

E. A. HARPER A T TORNE VS CONTROLLED EVAPORATION IN HEAT EXCHANGE ZONES This invention relates to a method for the indirect exchange of heat between at least three fluid streams. In one aspect this invention relates to heat exchange in a cryogenic separation process. In another aspect it relates to an improved helium recovery process and apparatus therefor.

In the recovery of helium or other products from natural gas by cryogenic separation the effectiveness of the recovery process is highly dependent upon the efficiency of the heat transfer steps. Therefore any improvement in the heat transfer steps will result in a substantial improvement of the helium recovery process.

In cryogenic separation a natural gas feed is separated into components by a process comprising liquefaction and then distillation of the liquid. One technique used to liquefy the natural gas feed involves self-cooling the feed by expansion and then passing a portion of the cooled stream in indirect heat exchange with the feed stream. By efficiently employing a series of heat exchangers the refrigeration effect produced thereby is accumulative permitting extremely low temperature reductions. In order to obtain a sufiiciently wide AT in the heat exchange step, it sometimes is necessary to supply additional coolant from an external source. In the past, this has been provided by an auxiliary nitrogen refrigeration cycle. The auxiliary refrigeration cycle increases the complexity of the system as well as the equipment costs which of course is an important consideration in any large scale process.

The purpose of the present invention is to provide coolant for refrigeration in a cryogenic process by utilizing existing equipment, thereby reducing the complexity of the system and the cost of equipment. The rich gas efi'luent containing C C hydrocarbons is generally further processed to recover additional commercial products, the most important being propane, butane and heavier components. This process employs a fractionator for selectively recovering the heavier components, e. g. C and heavier, and a condenser for furnishing reflux to the fractionator. It has been found that by recirculating a portion of the reflux stream through the heat exchanger in concurrent flow with the natural feed gas stream, and recombining the recirculate portion with the rich gas stream, the AT of the heat exchange step is increased. The increased AT is primarily due to the increased mass rate of flow of the refrigerant in indirect heat exchange with the feed stream. Thus by modifying existing equipment the efficiency of the heat exchange step is increased. In some systems the increased refrigeration effect afforded by this invention will be sufficient in and of itself. In others an auxiliary refrigeration system may be necessary but the capacity of the auxiliary system can be reduced owing to the increased refrigeration effect attributable to the present invention.

Accordingly, it is an object of this invention to provide an improved method for the transfer of heat between at least three fluid streams.

Another object of this invention is to provide an improved method for the indirect transfer of heat between at least three fluid streams wherein at least one of said fluid streams is a liquid and is at least partially vaporized during the heat exchange step.

A further object of this invention is to provide an improved helium recovery process.

A still further object of this invention is to improve the helium recovery process by modifying existing equipment.

These and other objects will become apparent to those skilled in the art from the following disclosure taken in conjunction with the accompanying drawings in which:

FIGS. 1, 2 and 3 are schematic representations of one embodiment of the inventive process and apparatus therefor; and

FIGS. 4 and 5 graphically illustrate the effect of the present invention on the helium recovery system shown in FIGS. 1, 2 and 3.

Referring to FIG. I, a natural gas feed comprising helium, nitrogen and C to C hydrocarbons is passed through conduit 11 into and through flow path 12 of heat exchanger wherein the feed gas is substantially cooled as hereinafter and more fully described. The partially condensed feed is passed through conduit 14 into liquid-gas separator 16. A liquid comprising predominantly C, to C hydrocarbons with some nitrogen is withdrawn from separator 16 via conduit 17.

The liquid withdrawn from separator 16 via conduit 17 is subjected to a pressure reduction-flash vaporization step by passing the liquid through valve 25 and into separator 18. A vapor at a reduced temperature and pressure is withdrawn from separator 18 via conduit 19 and passed to heat exchanger 10. A liquid at a reduced temperature and pressure is withdrawn from separator 18 via conduit 20, and introduced into exchanger 10 where it combines with the stream in conduit 19. The vapor-liquid mixture in combination with another vaporous stream, hereinafter described, is passed through heat exchanger 10 by means of flow path 21 in indirect heat exchange with the feed flowing through heat exchanger 10 via flow path 12 and other fluids flowing through heat exchanger 10. The liquid within flow path 21 is vaporized within heat exchanger 10 and rich gas is withdrawn from heat exchanger 10 via conduit 22. As described in detail below, the rich gas withdrawn via conduit 22 is passed to a fractionator system shown in FIG. 3 for further processing.

Vapors, which comprise substantially all of the helium and a major portion of the nitrogen contained in the feed gas stream, with the remainder being primarily C hydrocarbons with a small quantity of C to C hydrocarbons, are withdrawn from separator 16 and passed through the remainder of the system for cryogenic separation of the stream into products, e.g. helium, nitrogen, and C etc. The vapors are withdrawn via conduit 23 and passed through heat exchanger 24 via flow path 26. The vapor is partially condensed within heat exchanger 24 and a vapor-liquid mixture passed via conduit 27 to liquidvapor separator 28. A liquid, which is primarily C hydrocarbons together with substantially all of the C to C hydrocarbons passing through conduit 27, with a minor portion of the nitrogen, is withdrawn from separator 28 via conduit 29 and subjected to a pressure reduction-flash vaporization step by passage via valve 31 to separator 30 maintained at a substantially lower pressure.

Vapors are withdrawn from separator 30 via conduit 32. Liquid is withdrawn from separator 30 via conduit 34, introduced into the heat exchanger 24 wherein it joins with the stream in conduit 32 and a vapor-liquid mixture passed through heat exchanger 24 via flow path 33. The vapor-liquid mixture passed through heat exchanger 24 via flow path 33 is in indirect heat relationship with a fluid flowing through flow path 26 and other fluids flowing through heat exchanger 24, cooling the fluid flowing through flow path 26 and vaporizing the liquid in the vapor-liquid mixture flowing through flow path 33.

A vapor is withdrawn from heat exchanger 24 via conduit 36 and a major portion of the vapor passed through heat exchanger 10 via flow path 40. If desired, a small portion of the vapor in conduit 36 can be passed via conduit 37 and valve 38 to conduit 19 wherein the vapor is combined with the stream in conduit 19. The rate of flow of vapor through conduit 37 is controlled by a conventional flow recorder-controller 39 opening and closing valve 38 responsive to a rate of flow measurement in conduit 37 and a set point representative of a desired rate of vapor flow through conduit 37. A residue gas is withdrawn from heat exchanger 10 via conduit 41.

A vapor, comprising primarily helium, nitrogen, and C, hydrocarbons, is withdrawn from separator 28 via conduit 42 and passed through heat exchanger 43 via flow path 44 wherein said vapor is cooled. Cooled fluid is withdrawn from heat exchanger 43 via conduit 46 and passed through heat exchanger 47 via flow path 48 wherein the fluid is further cooled. A liquid-vapor mixture is withdrawn from heat exchanger 47 via conduit 49 and subjected to a pressure reduction-flash vaporization step by passing the mixture through valve 55 and into chamber 50 of column 51.

Chamber 50 also acts as a vapor-liquid separator. The liquid is withdrawn from chamber 50 via conduit 52 and passed through flow path 53 of heat exchanger 47 in indirect heat exchange with the fluid flowing through flow path 48. A slightly heated fluid is withdrawn from heat exchanger 47 via conduit 54 and passed to the upper region of chamber 56 of column 51. A stripping element 57 is positioned in chamber 56 below the entry therein of conduit 54 to provide for greater separation of the gas phase from the liquid phase.

Liquid is withdrawn from chamber 56 via conduit 58 and passed through heat exchanger 43 via flow path 61 in indirect heat exchange with the fluid flowing through flow path 44 and other fluids flowing through heat exchanger 43. A heated fluid is withdrawn from heat exchanger 43 via conduit 62 and flowed to separator 30.

A vapor, withdrawn from chamber 50 via conduit 63, is combined with the vapors withdrawn from chamber 56 via conduit 60 and the mixture is passed via flow path 64 through heat exchanger 65 wherein the said vapor mixture is cooled. A cooled liquid-vapor mixture is withdrawn from heat exchanger 65 via conduit 66 and passed into upper chamber 101 of column 67.

A helium stream is withdrawn from the top of separator chamber 101 via conduit 68 and passed through heat exchanger 65 via flow path 69 in in indirect heat exchange with fluids flowing through heat exchanger 65. A heated helium stream is withdrawn from heat exchanger 65 via conduit 80 and passed through heat exchanger 43 via flow path 81 in indirect heat exchange with fluids flowing through heat exchanger 43. Helium, further heated in heat exchanger 43, is withdrawn from heat exchanger 43 via conduit 82 and passed through heat exchanger 24 via flow path 83 in indirect heat exchange with fluids flowing through heat exchanger 24. Hellum at a higher temperature is withdrawn from heat exchanger 24 via conduit 84 and is further heated upon being passed through heat exchanger via flow path 86 in indirect heat exchange with fluids flowing through heat exchanger 10. A helium product stream is withdrawn from heat exchanger 10 via conduit 87.

A liquid is withdrawn from separator chamber 101 and passed to a lower chamber 102 via conduit 103. The liquid is subjected to a pressure reduction-flash vaporization by valve 104 thereby forming liquid and gaseous phases in chamber 102. A stripping element 105 is positioned in chamber 102 below the entry thereof to provide for a greater separation of the liquid and gas phases. A vapor is withdrawn from chamber 102 via conduit 106 and circulated through a helium recycle system (not shown) and returned to the upper chamber 101 via conduit 107.

A liquid comprising mostly nitrogen and C is withdrawn from chamber 102 via conduit 108 and passed to a nitrogen separation system. Referring to FIG. 2, the liquid in conduit means 108 is passed first through heat exchange means 109 via flow path 110 and then through heat exchanger 111 via flow path 112, with conduit 113 interconnecting paths 110 and 1 12.

In passing through heat exchangers 109 and 111, the liquid stream is preheated, forming a vapor liquid mixture which enters a nitrogen separator 114 via conduit 115. Vapors, the composition of which is predominantly nitrogen, are passed to a nitrogen recycle compressor (not shown) via conduit 116. The vapors are compressed and returned to the system shown in FIG. 2 via conduit 117. From conduit 117, the vapors pass through heat exchanger 109 via flow pass 118 in indirect heat exchange with fluid flowing through flow pass 110. in the heat exchanger 109, the vapors passing through flow pass 118 are cooled forming a liquid gasvapor mixture which is passed into separator 119 via conduit 120 after being subjected to a pressure-reduction-flash vaporization step by valve 121. A liquid is withdrawn from separator 119 and recycled to the nitrogen separator 114 via conduit 122, the flow in conduit 122 being controlled by valve 123.

Vapor, which is substantially nitrogen with a very small amount of C is withdrawn from separator 119 via conduit 124 and passed successively through heat exchangers 43, 24 and 10 via flow passes 125, 126 and 127, respectively (see FIG. 1). Conduits 128 and 129, respectively, interconnect flow passes and 126, 126 and 127. The predominantly nitrogen vapors are withdrawn from the first heat exchanger 10 via conduit 130 and are passed to a nitrogen compressor system (not shown).

In order to provide a medium for preheating the stream entering the nitrogen separator 114, a portion of the stream in conduit 60 is diverted to heat exchanger 111 via conduit 131. The liquid-vapor mixture is passed from conduit 131 through the heat exchanger 111 via flow pass 132 in indirect heat exchange with the fluid flowing through flow pass 112. The cooled liquid-vapor mixture is passed from heat exchanger pass 132 to conduit 66 via conduit 133.

A liquid which is predominantly C hydrocarbons and nitrogen is withdrawn from the bottom of separator 114 via conduit 70 and passed through heat exchange 65 via flow path 71 in indirect heat exchange with a fluid flowing through flow path 64. Within heat exchanger 65 the liquid feed is vaporized and passed from heat exchanger 65 via conduit 72 to heat exchanger 43. The lean gas is passed through heat exchanger 43 via flow path means 73 in indirect heat exchange with fluids flowing through heat exchanger 43. Heated lean gas is withdrawn from heat exchanger 43 via conduit 74 and passed through heat exchanger 24 via flow path 76 in indirect heat exchange with the fluids flowing through heat exchanger 24. A further heated lean gas is withdrawn from heat exchanger 24 via conduit 77 and passed through heat exchanger 10 via flow path 78 in indirect heat exchange with the feed material flowing through flow path 12 and other fluids flowing through heat exchanger 10. Lean gas, heated in heat exchanger 10, is withdrawn from heat exchanger 10 via conduit 79.

The rich gas effluent in conduit 22 comprising mainly C C; hydrocarbons with lesser amounts of C -C hydrocarbons is further processed in the plant shown in FIG. 3.

The plant shown in FIG. 3 comprises principally a fractionator 13S and a reflux condenser 136. The fractionator 13S functions to separate the more volatile fractions, C C and some C;, from the mixture, recovering the lower fractions C -C as kettle product.

The rich gas in conduit 22 passes to a compressor 137 where the stream is pressurized and passed through a cooler 138 via conduit 139. From the cooler 138, a liquid-vapor mixture is passed to a heat exchanger 139a via conduit 140. The stream passes through flow pass 140a in heat exchanger 139:: further cooling the mixture. The cooled vapor-liquid mixture is then passed via conduit 141 into fractionator from which overhead vapors comprising C -C hydrocarbons are withdrawn through conduit 142.

The liquid, comprising mainly C -C is withdrawn from the fractionator 135 via conduit 143 and may be further processed or may be used as is. The overhead vapor is passed through the partial condenser 136 via flow pass 144 which is in indirect heat exchange with a coolant flowing in pass 145. The cooled vapor-liquid mixture passes from condenser 136 to a separator 146 via conduit 147. Note that the separator 146 is the third separation zone for rich gas stream, separator 16 being the first and separator 18, the second. Vapors comprising mainly C, and C hydrocarbons are withdrawn from separator 146 via conduit 153 and, after passing through heat exchanger 139a via flow pass 154, are returned to the residue gas line 41 via conduit 155. Fractionator reflux liquid comprising intermediate hydrocarbon range (mainly C and C;, with some C is withdrawn from separator 146 via conduit 148 and pumped by pump 149 through conduit 150. A portion of the reflux stream is returned to the fractionator 135 via conduit 151 and the remainder is diverted through conduit 152 to the helium separation system shown in FIG. 1. The portion of the reflux stream returned to the helium separation system can be combined with the feed stream in conduit 11.

Preferably, however, the reflux stream is passed separately through the heat exchanger via conduit 156 in concurrent flow with the feed stream in pass 12 and in indirect heat exchange with streams in passes 21, 40, 78, 86, and 127.

Since the temperature of the liquid stream in conduit 152 is less than the feed stream entering heat exchanger via conduit 11, the entrance of flow pass 156 in heat exchanger 10 is located such that the liquids from conduit 152 enter in indirect heat exchange at a point where the feed stream in pass 12 has been cooled to the temperature of the liquid in conduit 152. The cooled stream is then passed from flow pass 156 to valve 157 via conduit 158. The stream is subjected to a pressure reduction-flash vaporization through valve 157 and the resulting liquid-vapor mixture is passed to separator 18 via conduit 159. An intermediate hydrocarbon range of the fractionator overhead vapors are thus combined with the rich gas stream at a point upstream of separator 18.

Introducing the portion of the reflux stream into the helium separation system at a point upstream of the separator permits thorough mixing of the combined streams prior to vaporization in the heat exchanger 10. As illustrated below, the combining of the fractionator overhead vapors (reflux stream) with the rich gas stream and the subsequent passing of the combined streams through heat exchanger 10 via flow pass 21 provides for an increase AT in the heat exchange zone 10. The fractionator 135 and condenser are operated at conditions to control the hydrocarbon range refluxed to the fractionator which in turn provides the proper composition for use in the helium separation system. The selected hydrocarbon range to effect the increased AT will depend upon the operating characteristics of the separation system, particularly the constitution of the rich gas stream in conduit 17 and the temperature range in heat exchange pass 10.

In this preferred embodiment describing cryogenic separation of helium, the feed stream in pass 12 of heat exchanger 10 is cooled from about 97 F. to about 80 F. Without the benefit realized by the present invention, at AT indicated by arrow 166 of FIG. 5 between the counterflowing streams in heat exchanger 10 is small (about 7 F. as shown in FIG. 5). Thus in order to increase the AT and hence heat exchanger efficiency, the portion of the reflux stream returned to the helium recovery system must be of such amount to substantially increase the mass rate of flow of refrigerant through pass 21. Furthermore, the composition of the portion returned must be compatible with the stream in conduit 17. Still further, the facilities must provide for adequate mixing of the two streams prior to the vaporization step occurring in pass 21. It has been found that increasing the rate of refrigerant flowing through pass 21 by about -25 weight percent owing to the addition of the portion of fractionator overhead vapors (composed mainly ofC,-C the AT can be increased to about 12 F.

While the present invention has been described in connection with a helium recovery system, it should be realized that for different cryogenic systems separating other components such as hydrogen or natural gas, the constitution of the rich gas effluent would be different requiring a different constitution of reflux stream to effect the increased AT of heat transfer in zone 10.

The reflux stream returned to the separation system can be returned to the rich gas stream at any point between the separator 16 and heat exchanger 10, but preferably is returned at a point upstream of separator 18 to insure thorough mixing ofthe stream.

As mentioned above, it has been found that by returning a portion of the fractionator overhead vapors from the fractionator system to the helium system, the efficiency of the heat exchange step in the latter system is greatly improved. More specifically, the AT of the heat exchanger 10 is increased throughout the heat exchange zone. A particular feature of this invention is that the improved heat exchange efficiency is achieved by interconnecting existing equipment of associated systems.

The effect of this invention on the heat exchanger 10 of the helium separation system is graphically illustrated in FIG. 4. Line 160 represents the temperature of the feed stream and fractionator overhead vapor stream passing through flow pass 12 and 156 of heat exchanger 10. Arrow 161 indicates the point at which the fractionator overhead vapor stream enters into heat transfer in exchanger 10. Line 162 represents the temperature of the product streams passing through flow passes 21, 40, 78, 86 and 127 of heat exchanger 10. As illustrated in this example, the AT of the counterflowing streams is about 12 F. as shown by vertical lines 165. FIG. 5 represents the heat exchange effect of the same system without the improvement provided by this invention. That is to say, a portion of the condensed overhead vapors of the fractionator system were not returned to the helium separator system. Line 163 represents the feed stream passing through flow pass 12 of heat exchanger 10 and line 164 represents the product streams passing through flow passes 21, 40, 78, 86 and 127 of heat exchanger 10. As illustrated, the AT (AT near the cold end is large whereas the AT (AT near the hot end is small (about 5 F.). Of course, the degree of improvement will depend upon the operating conditions of the helium separation system and the fractionator system. The graphs presented in FIGS. 4 and 5 were calculated using the following inlet temperatures at predetermined rates of flow.

Outlet temperature. F.

Inlet Without With tempel'ainvention invention ture, F.

Flow pass:

12 (Feed) J7 80 156 (fractionator overhead reflux) 20 80 21 (rich gas t )1 84 40 (residue gas) 92 91 84 78 (lean gas 92 91 84 86 (helium) 92 91 84 127 (nitrogen). 92 91 84 While the preferred embodiment of this invention has been described in particular detail, it should be emphasized that modifications and variations may be made therein without departing from the scope and spirit of the invention as set forth in the appended claims.

What is claimed is:

1. In a process including the steps of passing a feed gas stream containing C -C hydrocarbons through a heat exchange zone thereby forming a liquid-vapor mixture, separating said liquid-vapor mixture into components including a product stream, a gas stream containing mostly C, hydrocarbons, and a rich gas stream containing mostly C,6 (1C hydrocarbons, forming a vapor-liquid mixture in each of said product streams, passing said liquid-vapor mixtures separately through said heat exchange zone in indirect heat exchange with said feed gas stream, said component streams vaporizing in said heat exchange zone to cool said feed gas stream, the improvement comprising passing said rich gas stream from said heat exchange zone to a fractionator, recovering a liquid containing C3-C6 hydrocarbons from said fractionator, withdrawing a vapor containing C -C hydrocarbons from said fractionator, partially liquefying said vapor, separating vapor and liquid of said vapor-liquid mixture, said vapor containing mostly C and C hydrocarbons, refluxing a portion of said liquid containing mostly C and C hydrocarbons to said fractionator, passing the remainder of said liquid through said heat exchange zone in indirect heat exchange with said component streams thereby cooling said liquid, and

combining said liquid and said rich gas stream at a point upstream of said heat exchange zone.

2. The invention as recited in claim 1 wherein said liquid leaving said heat exchange zone is subjected to a pressure reduction flash vaporization forming a liquid-vapor mixture, and said liquid-vapor mixture is passed into said rich gas stream.

3. The invention as recited in claim 2 wherein said product stream includes helium.

4. The invention as recited in claim 3 wherein said product stream further includes nitrogen.

5. The invention as recited in claim 4 wherein said liquid combined with said rich gas is from about percent to about percent by weight of the total combined stream.

6. In a process including the steps of passing a natural gas stream through a heat exchange zone thereby forming a liquid-vapor mixture, passing said mixture into a first separation zone, withdrawing a vapor from said first separation zone, separating said vapor withdrawn from said first separation zone into product streams by liquefaction and selective distillation, passing said product streams separately through said heat exchange zone in indirect heat exchange with said natural gas stream, withdrawing a liquefied rich gas stream containing mostly C C and C hydrocarbons with lesser amounts of C and heavier hydrocarbons, from said first separation zone, subjecting said liquefied rich gas stream to a pressure-reduction flash vaporization thereby forming a liquid-vapor mixture, passing said mixture into a second separation zone, separately withdrawing a vapor and a liquid from said second separation zone, combining said liquid and vapor withdrawn from said second separation zone, passing said combined vapor and liquid through said heat exchange zone in indirect heat exchange with said natural gas stream wherein said liquid is vaporized to cool said natural gas stream, withdrawing a vaporized rich gas stream from said heat exchange zone, the improvement comprising liquefying a portion of said vaporized rich gas stream thereby forming a liquid-vapor mixture, passing said mixture into a fractionator, recovering a kettle product containing C and heavier hydrocarbons from said fractionator, withdrawing a vapor containing C and lighter hydrocarbons from said fractionator, partially condensing said vapor withdrawn from said fractionator, passing said partially condensed vapors into a third separation zone, withdrawing a liquid from said third separation zone, passing a portion of said liquid withdrawn from said third separation zone through said heat exchange zone in indirect heat exchange with said product streams and said rich gas stream thereby cooling said liquid, and passing said a portion of said liquid into said rich gas stream upstream of said heat exchange zone, and refluxing the remainder of said liquid withdrawn from said third separation zone to said fractionator.

7. The invention as recited in claim 6 wherein said product streams include helium, said rich gas stream includes C -C hydrocarbons, said liquid withdrawn from said fractionator includes C -C hydrocarbons, and said liquid withdrawn from said third separation zone includes C C and C hydrocarbons.

8. The invention as recited in claim 6 and further comprising the step of subjecting said liquid withdrawn from said heat exchange zone to pressure-reduction flash vaporization prior to being passed into said rich gas stream.

9. The invention as recited in claim 8 wherein the liquidvapor mixture resulting from said pressure-reduction flash vaporization is passed to said rich gas stream upstream of said second separation zone.

10. The invention as recited in claim 9 wherein the amount of said portion of said liquid withdrawn from said third separation zone passed through said heat exchange zone comprises from 20 to 25 weight percent of the stream passing to the second separation zone when combined with the rich gas I12, conduit UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTICN Patent No. 3,607,733 D t September 21, 197

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, lines 59-60, "016G036" should read C -C6 Column 8, lines 31., and 35, "112, conduit" should read stream Signed and sealed this 7th day of March 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER,JR.

Commissioner of Patents Attesting Officer 

2. The invention as recited in claim 1 wherein said liquid leaving said heat exchange zone is subjected to a pressure reduction flash vaporization forming a liquid-vapor mixture, and said liquid-vapor mixture is passed into said rich gas stream.
 3. The invention as recited in claim 2 wherein said product stream includes helium.
 4. The invention as recited in claim 3 wherein said product stream further includes nitrogen.
 5. The invention as recited in claim 4 wherein said liquid combined with said rich gas is from about 20 percent to about 25 percent by weight of the total combined stream.
 6. In a process including the steps of passing a natural gas stream through a heat exchange zone thereby forming a liquid-vapor mixture, passing said mixture into a first separation zone, withdrawing a vapor from said first separation zone, separating said vapor withdrawn from said first separation zone into product streams by liquefaction and selective distillation, passing said product streams separately through said heat exchange zone in indirect heat exchange with said natural gas stream, withdrawing a liquefied rich gas stream containing mostly C1, C2, and C3 hydrocarbons with lesser amounts of C4 and heavier hydrocarbons, from said first separation zone, subjecting said liquefied rich gas stream to a pressure-reduction flash vaporization thereby forming a liquid-vapor mixture, passing said mixture into a second separation zone, separately withdrawing a vapor and a liquid from said second separation zone, combining said liquid and vapor withdrawn from said second separation zone, passing said combined vapor and liquid through said heat exchange zone in indirect heat exchange with said natural gas stream wherein said liquid is vaporized to cool said natural gas stream, withdrawing a vaporized rich gas stream from said heat exchange zone, the improvement comprising liquefying a portion of said vaporized rich gas stream thereby forming a liquid-vapor mixture, passing said mixture into a fractionator, recovering a kettle product containing C3 and heavier Hydrocarbons from said fractionator, withdrawing a vapor containing C3 and lighter hydrocarbons from said fractionator, partially condensing said vapor withdrawn from said fractionator, passing said partially condensed vapors into a third separation zone, withdrawing a liquid from said third separation zone, passing a portion of said liquid withdrawn from said third separation zone through said heat exchange zone in indirect heat exchange with said product streams and said rich gas stream thereby cooling said liquid, and passing said a portion of said liquid into said rich gas stream upstream of said heat exchange zone, and refluxing the remainder of said liquid withdrawn from said third separation zone to said fractionator.
 7. The invention as recited in claim 6 wherein said product streams include helium, said rich gas stream includes C1-C6 hydrocarbons, said liquid withdrawn from said fractionator includes C3-C6 hydrocarbons, and said liquid withdrawn from said third separation zone includes C1, C2, and C3 hydrocarbons.
 8. The invention as recited in claim 6 and further comprising the step of subjecting said liquid withdrawn from said heat exchange zone to pressure-reduction flash vaporization prior to being passed into said rich gas stream.
 9. The invention as recited in claim 8 wherein the liquid-vapor mixture resulting from said pressure-reduction flash vaporization is passed to said rich gas stream upstream of said second separation zone.
 10. The invention as recited in claim 9 wherein the amount of said portion of said liquid withdrawn from said third separation zone passed through said heat exchange zone comprises from 20 to 25 weight percent of the stream passing to the second separation zone when combined with the rich gas 112, conduit 